Visual acuity is the acuteness or sharpness of a person's vision. In the 19th century, Franciscus Donders defined visual acuity as the magnification needed to perform as well as the standard eye. It was believed that the standard eye could recognize letters that subtend an angle of 5′ of arc at a given distance, usually 20 feet. The magnification needed was calculated first by dividing the size of letters visualized by a patient by the size of letters visualized by the standard eye. The reciprocal of the magnification needed equaled the visual acuity. In 1862, Hermann Snellen and Donders introduced the letter chart, also known as the Snellen chart, which is still used today to measure visual acuity. The traditional Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter and each subsequent row has additional letters; each row decreases in size. A person taking the test covers one eye and reads aloud the letters of each row. The smallest row that can be read accurately indicates the visual acuity in that eye. The letters on an acuity chart are formerly known as optotypes. Wall-mounted Snellen charts are inexpensive and are used to approximate visual acuity, such as, for children in a school nurse's office.
Sometimes however, visual acuity approximations are insufficient and more careful assessments are needed, for example, during an eye examination performed by a doctor evaluating a patient's diabetic retinopathy. More exact visual acuity tests can be performed with equipment such as computers. For example, U.S. Pat. No. 8,240,851 (Reichow et al.) discloses a computerized system for testing a person's visual ability including a presenting component, an input component arranged to receive input provided by a patient, and a processing component configured to process the user's input. Others use the logarithm of the minimum angle of resolution (log MAR) to measure visual acuity. In these tests, the sizes of the letters progress systematically in geometric progression. In contrast to the Snellen chart, the space between lines and letters change in proportion so that the effect of contour interaction is constant. Log MAR charts include five letters on each line as opposed to the Snellen chart.
Measuring visual acuity requires knowing the size of the letters or optotypes tested, the distance between the patient and the letters or optotypes tested, and the patient's ability to accurately perceive the letters or optotypes. The near vision determined with respect to reading a text is also referred to as reading acuity. To test reading acuity, it is further required to know the speed at which the patient can accurately read the tested sentences.
European Patent Application No. EP07815184 discloses a device for determining the near vision acuity for reading text and the recognition of graphic illustrations by a patient. The teachings of European Patent Application No. EP07815184 are incorporated herein by reference in its entirety. The disclosed device includes a presentation surface on which a number of texts or various sizes can be displayed, a microphone for recording feedback from the patient when reading the texts, and a computer system set up for monitoring the reading process based on the microphone recordings and for determining the patient's near vision or reading acuity in relation to the still legible font size. The patented invention discloses a proposed means for measuring the reading distance freely selected by the patient to the text presented to the patient on the presentation surface. The computer system of the invention disclosed includes cameras to detect the presence of the reader's face by detecting a sensor placed in between the reader's eyes via infrared technology. The cameras are manually calibrated to detect how far the sensor on the reader's face is from the text/graphics using a ruler. This manual process is tedious and time consuming. Unfortunately, Reichow et al. discussed above do not disclose a means for automatically calibrating a distance measured between a patient and the presentation component of the device of the invention.
Therefore, there is a long-felt need for an improved method and apparatus for testing near vision acuity with automated calibration. There is also a long-felt need for a system and method for determining reading acuity that randomizes the sentences presented on a screen and adjusts the reading rate calculation to accommodate for misread words.